Electrostatic and electrophotographic printing involves developing a latent electrostatic image with charged toner particles loaded onto an imaging drum and transferring them onto a substrate or a print substrate, particularly in the form of sheets or in the form of a continuous conveyor belt. As an example, in four-color printing, four latent images in the four-color separations (cyan, magenta, yellow, and black) are transferred to the substrate successively and in register on top of one other. In particular, the finished single color or multicolored latent image is then fused onto the substrate by a fusing device. This customarily takes place by a heatable fusing roller, which is rolled onto the toner image. The toner is heated up above its glass transition temperature, and thus melted, and simultaneously incorporated under pressurization into the substrate to which it is fused after it has been cooled. Adjacent toner particles are thereby combined, which finally form a polymer layer on the substrate.
Alternatively, fixing can be performed in other ways, for example, by exposing the toner and the substrate to which the toner is to be fused to microwaves. Such microwaves heat water in the printing substrate to which the toner is to fuse. This water heats the substrate. The substrate then transfers heat into the toner to cause the toner to reach the glass transition temperature. However, it has been difficult in the past for such microwave fusing systems to operate in the optimal fusing area (fusing window) between a fusing extent that provides an inconsistent gloss and blister formation of the toner on the substrate. This window is very narrow, in particular with the use of glossy-coated paper as the substrate.
To address such problems, U.S. Pat. No. 6,909,871 entitled Method and Device for Fusing Toner Onto a Substrate, filed by Behnke et al. on Apr. 14, 2003, proposes combining these methods and describes a fusing method wherein a heated fusing roller is used to heat the toner, under pressure, to a temperature that is greater or equal to a glass transition temperature, and additionally applying microwaves heat the toner on a substrate. In order to achieve a better energy input, Behnke et al. proposes that the substrate can be moistened before microwaves are applied to the substrate and suggests applying 100 degree C. hot steam to the substrate. Behnke et al. also suggests that the substrate could preferably be moistened on both sides, in order to avoid stressing and bending of the substrate and notes that one advantage of this technique is that the substrate carrying the toner can be warmed by the heat from the steam.
However, while the system that is described in Behnke et al. is useful for many purposes, there are many complications associated with the use of contact fusing such as is done with heated rollers or belts and in many applications fusing without using a contact surface such as a roller or belt is preferred.
Thus, what is still needed in the art are methods and printers that enable effective non-contact fixing of toner.